Frequency discriminating electric network



Sept. 14, 1937. H. L. OURA 2,093,248

FREQUENCY DISCRIMINATING ELECTRIC NETWORK Filed Sept. 50, 1953 IWVf/VTOB 195g: 70 L. 0054 5y 00m Patented Sept. 14, 1937 PATENTFREQUENCY DISCRIMINATING ELECTRIC NETWORK Hector Leslie Oura,Hillingdon, England, assignor to Electric and Musical IndustriesLimited,

Middlesex, Britain England,

a company of Great Application September 30, 1933, Serial No. 691,599 InGreat Britain October 3, 1932 8 Claims.

The present invention relates to frequency discriminating electricalnetworks, and more particularly but not exclusively to networks of thetype employed in audio-frequency amplifying arrangements and the likefor purposes of tone control. Networks of this type usually comprise oneor more variable elements, whereby the frequency discriminating propertyof the network may be adjusted.

Audio-frequency tone controls have numerous applications in modernwireless receivers; they are employed, for example, for correcting forside band attenuation taking place in the radiofrequency circuits of thereceiver, for compensating for imperfections in the frequency responseof the loudspeaker, and for other purposes.

It is an object of the present invention-to provide frequencydiscriminating electrical networks of the type referred to which aresimple to construct, and can readily be embodied in an audiofrequencyamplifying arrangement.

. It is a further object of the invention to provide a frequencydiscriminating electrical network which may be adjusted to have any one,of a number of frequency characteristics within limits at which lowfrequencies are favoured at the expense of high frequencies, or highfrequencies are favoured at the expense of low frequencies.

The invention will be described by way of example with reference to theaccompanying drawing, in which Figures 1 and 2 show diagrammatically twoforms of network according to the present invention and Fig. 3 showsdiagrammatically a form of uni-control operating means embodying theinvention Referring to Fig. 1, the primary Winding l of a transformer 2is tapped at a point 3, and a variable resistance 4 is connected inparallel with one of the two portions so formed. A variable condenser 5is connected in parallel with the secondary winding 6 of the transformer2. The resistance 4 and the condenser 5 are preferably ganged togetherso that they may be operated by one control member as indicated by thedotted line shown, and the arrangement is made such 1 that, startingfrom a midposition, rotation of the control member in one direction, forexample, causes variation only in thevalue of the resistance 4, whilerotation thereof in the opposite direction causes variation in thecapacity of the condenser 5, but does not vary the value of theresistance 4.

This may be accomplished, for example, by suitably shaping the vanes ofthe condenser 5,

and by arranging that when the movable contact of the resistance 1reaches the position in which the resistance in circuit is a maximum,the movable contact then tracks over an element of good conductivityconnected in series with the resistance element, so that when themovable contact passes the position of maximum resistance, substantiallyno further change of resistance occurs. It is further arranged thatwhile the movable contact tracks over the highly conducting element, thecapacity of the condenser 5 is varied, the capacity remaining constantat a minimum value, which may be substantially Zero, while the movablecontact tracks over the resistance element itself. Such an arrangementis shown in Fig. 3 in which the movable arm H is shown at one end ofresistor l, the movable plate IU of condenser 5 being secured to thesame operating means, shown as a shaft l2, to which arm II is secured.l3 denotes the fixed condenser plate or plates. It will be seen thatrotation of shaft I? to the left causes arm I l to slide over theconductive member it While the capacity of con denser 5 increases to itsmaximum value. Upon rotation of shaft l2 in the opposite direction,

however, the capacity of the condenser remains 2 substantially the samewhile the value of resistor 4 is decreased.

Alternatively, it may be arranged that when the value of the resistance4 reaches a maximum, the resistance element itself is open-circuited;variation of the capacity of the condenser 5 then takes place only whilethe resistance element is open circuited, and this capacity is arrangedto remain constant at a minimum value, such as substantially zero forexample, while variation in the value of the resistance 4 is takingplace. This arrangement may be had by simply omitting the track orconductive member M of Fig. 3. Methods other than those described ofobtaining independent variation of the resistance 4 and the capacity ofcondenser 5 by means of one control member will be apparent to thoseskilled in the art.

The arrangement illustrated in Fig. 2 is a modification of that shown inFig. l. The variable condenser 5 which in Fig. 1 is connected inparallel with the secondary winding 5 of the transformer 2 is replacedin Fig. 2 by a fixed condenser l in series with a variable resistance 8.

Adjusting means associated with resistance 4 are l preferably ganged tosimilar means associated with resistance 8, one control member beingprovided as indicated by the dotted line shown whereby simultaneousvariation of both resistances may be obtained. The arrangement is Iby-passing action of the condenser I.

preferably such that as the resistance 4 is increased, the resistance 8is decreased, and vice versa. V

The network illustrated in Fig. 2 is provided with two input terminals 9and 9. Connected in series between the terminal 9' and the upper end ofthe winding I is a resonant filter circuit comprising an inductance coilill in parallel with a condenser II, this circuit being tuned to afrequency against whichitis desired that the net- 7 work shoulddiscriminate. When the network is associated with a low'frequencyamplifier, the

circuit In, H may serve to provide a sharp high frequency cut-off, theamplifier and other apparatus with which it is associated being arrangedto have little response above the required cut-off the secondary winding6 of the transformer 2 tend'to be reduced in intensity on account of theAs the valueof resistance 8 is increased, this action is reduced, andthe value of the resistance I is also reduced. The latterresistance anda portion of the winding I constitute an inductive circuit.

As the resistance 4 is decreased, the impedance'of this inductivecircuit is decreased, and the effective inductance of the primarywinding 1 is also decreased. The transformer accordingly becomes lessefficient in the transmission of the lower frequencies, which thus tendto be suppressed. I

It will be seen that by varying simultaneously the resistances d and 8,the frequency response of the whole network may be varied progressivelyfrom a response which falls from a maximum at the lower frequencies, toone which rises to a maximum atthe higher frequencies.

It is to be noted that the frequency discriminating networks describedabove are given by way of illustration only, and many variations withinthe scope of this invention, as defined in the appended claims, willreadily occur to those skilled in the art; for example, in place of thetransformer employed in the arrangements described above, anauto-transformer or like device may be employed. Furthermore, thenetworks according to this invention are not limited in theirapplication to providing tone-control in broadcast receivers, but mayfind many other uses in the electroacoustic field.

I claim:

1. An electrical network for transmitting a wide band of audio frequencycurrents comprising the combination of a transformer having a primarywinding and a secondary winding, a

variable resistor connected between an intermediate point and one end ofone of said windings, a circuit having a capacitative reactanceconnected in shunt across the other of said windings, a directconnection between the low potential ends of said windings whereby saidends are maintained at the same potential, and a single adjusting meansarranged to varythe impedance of said circuit and the value of saidresistor.

2. Apparatus constituting a frequency discriminating electricalnetworkjcomprising an audio single adjusting-means for varying both thevalue of said resistor, and the capacity of said condenser. V

3. Apparatus constituting a frequency discriminating. electricalnetwork, comprising a transformer having a primary winding and a secondary winding, one of said windings being divided by means of a tapping.into two portions, a vari able resistance element connected in parallelwith one of said portions and constituting therewith a circuit having aninductive reactance, a circuit having acapacitative reactance andcomprising a condenser in series with a variable resistance elementconnected in parallelwith the other-of said windings, and a uni-controladjusting means for varying the resistance of both said elements.

4. Apparatus constituting an audio frequency discriminating electricalnetwork,zcomprising an 5. Apparatus constituting an 'audiof frequency.

discriminating electricalgnetwork, comprising an audio frequencytransformer having a primary winding and a secondary winding, a circuithaving an inductivereactance and comprisingavariable resistance elementassociated withoneg'of said windings, a circuitincludingaJVariabIewrevided into two portions, a variable resistance elementconnected in parallel with one of said portions and constitutingtherewith a circuit having an inductive reactance, a variable condenserconnected in parallel with the other of said-windings, adjusting meansfor varying the resistance of said element and the capacity of saidcondenser, and coupled to said adjusting means an operating member forvarying,over a part'oi its range of movement,the resistance of saidelement and, over another part of said range, the capacity of saidcondenser.

7. Apparatus constituting a frequency discriminating electrical network,comprising an input terminal and an output terminal, a transwith one ofsaid portions and constituting there- 7:5,

former having a, primary wi d a ailysecdnm ;,7 0

with a circuit having an inductive reactance, a circuit having acapacitative reactance connected in parallel with the other of saidwindings, an inductance coil in parallel with a condenser connectedeffectively in series between said input terminal and said outputterminal, and a single adjusting means for varying the impedance of bothsaid reactive circuits consecutively.

8. Apparatus constituting a frequency discriminating electrical network,comprising an audio frequency transformer having a primary winding and asecondary winding, one of said windings being divided into two portions,a variable resistance element having its ends conductively connected inparallel with one of said portions and constituting therewith a circuithaving an inductive reactance, a variable condenser connected inparallel with the other of said windings, and a uni-control adjustingmeans for varying both the value of said resistance and the capacity ofsaid condenser.

HECTOR LESLIE OURA.

